This invention relates generally to layered films formed of thermoplastic material and methods for their manufacture and, more particularly, to layered films and their method of manufacture, wherein the film is defined by a plurality of superposed layers and wherein the molecular chains of the material forming pairs of adjacent layers are oriented in certain directions which respectively cross each other.
Layered films formed of thermoplastic material are known and are generally manufactured by the so-called "blown-film" extrusion process. More particularly, according to such known processes, raw material in the form of a particular thermoplastic material suitable for the purpose intended in solid form, such for example as pellets, powder or amorphous particles, is mechanically transformed by mechanical working, such as through the action of a rotary screw associated with an extrusion machine, and through the application of heat provided, for example, by electric resistance heating, into a viscous mass at least at the softening point of the raw material. The viscous mass is conveyed to an annular die cavity or material forming opening of an extrusion die or matrix which forms the material into a two layered tubular segment defining an interior space together with the rotary matrix and a pair of stretching and rolling cylinders upstream thereof to which the tubular segment is directed, into which interior space a predetermined amount of gas is injected or "blown" to give the tubular segment a desired width. Examples of such techniques are disclosed in U.S. Pat. Nos. 3,635,634; 4,009,975; 4,011,128; and 3,891,374.
In such known technique, the stretching cylinders are rotated at a rate or velocity which is compatible with and determined by the rate at which the tubular segment discharges from the die opening so as to continue to feed the material at the same rate as that at which it is discharged from the matrix. In this manner, the tubular segment is shaped so as to have the desired thickness. The thus flattened tubular segment thereupon passes between a set of rolling cylinders and is thereafter subjected to further treatments and procedures so as to impart to the finished product any particular characteristics required for its use.
Further, in such known methods, the die may be rotated for the purpose of avoiding irregularities in the thickness of the formed film and to thereby avoid the formation of crests and valleys on any roll of the layered film which may be formed. Such crests and valleys would form, for instance, where film extruded from a non-rotating die having zones of greater and lesser thickness, was wound so that such zones coincide in overlapping turns. In order to achieve the avoidance of such thickness irregularities in the known processes being described, the matrix is conventionally rotated at a rate in the range of between 1/10 and 1/2 rpm.
In the "blown film" extrusion technique described above, the film formed by the matrix is desirably stretched by stretching cylinders in a balanced manner to obtain a favorable balance of strength for the film in the longitudinal and transverse directions. Such a stretching of the formed film results in a unidirectional orientation of the molecular chains of the thermoplastic material which depends on various factors including the particular material being formed, the particular working conditions and, to a greater extent, on whether the material is of the long or short chain type.
In this connection, the opening between the cylinders is normally variable, the extent of the opening being limited at its upper end to avoid wrinkling of the film which is a particular danger in the case of rigid thermoplastic material, such as high density polyethylene, polypropylene, etc. and at its lower end by the possibility of presenting an obstruction to the passage of the film where the opening is too narrow for the pressures and temperatures involved in the particular process.
Prior art techniques of the type described above are generally characterized by the provision of equipment for cooling and solidifying the molten material upon the latter discharging from the die opening. The tubular segment is then directed over one or more rotating and fixed mandrels whereupon additional apparatus reheats the previously solidified tubular segment, the mandrels then imparting an angular orientation to the molecular chains in the reheated regions thereof. This procedure has been utilized for the reason that it has conventionally been the understanding that oriented molecular chains in thermoplastic material could not be effectively obtained when the material is in a molten state. Thus, these conventional techniques require the provision of equipment for cooling the molten material and for reheating the same, as well as rotating and fixed mandrels, which of course renders the apparatus relatively complicated and expensive in manufacture. Further, the width of the film produced by such conventional techniques was necessarily determined by the dimensions of the mandrels and the die being utilized.
The presently known fabrication technique described above has not proved to be entirely satisfactory for other reasons. Thus, in view of the increase in need for automatic and semi-automatic packing processes which require large quantities of materials, the development of a film having a high heat-sealing capacity has become essential. Such a film cannot be obtained using the fabrication technique described above. Further, the films provided by known techniques do not have optimum toughness or tensile, elongation and rupture strength.